Question

Which of the following molecules is chiral in nature ?

• A. Propan–2–ol
• B. Butan–2–ol
• C. 1–Bromobutane
• D. 2–Bromopropane

Hint:

To identify chirality quickly in exams, look for a "fork" in the carbon chain where the groups on either side of the functional group are of different lengths or compositions (like Methyl vs Ethyl).

Answer 1

The Correct Option is B

Concept: Chirality is a fundamental property of three-dimensional objects. In organic chemistry, a molecule is said to be chiral if it cannot be superimposed on its mirror image.

• The most common cause of chirality is the presence of an asymmetric carbon atom (also known as a chiral center or stereocenter).

• An asymmetric carbon is a \(sp^3\) hybridized carbon atom that is covalently bonded to four different atoms or groups of atoms.

• If a molecule possesses a plane of symmetry or a center of symmetry, it is achiral.
Step 1: Detailed structural analysis of Propan-2-ol.
The structure of Propan-2-ol is \( \text{CH}_3-\text{CH(OH)}-\text{CH}_3 \). Let us examine the central carbon (C-2):

• It is bonded to a hydrogen atom (\(-\text{H}\)).

• It is bonded to a hydroxyl group (\(-\text{OH}\)).

• It is bonded to a methyl group (\(-\text{CH}_3\)) on the left.

• It is bonded to a methyl group (\(-\text{CH}_3\)) on the right.
Since two of the four groups (the two methyl groups) are identical, the carbon atom is not asymmetric. Consequently, the molecule has a plane of symmetry passing through the C-H and C-OH bonds, making it achiral.

Step 2: Detailed structural analysis of Butan-2-ol.
The structure of Butan-2-ol is \( \text{CH}_3-\text{CH(OH)}-\text{CH}_2-\text{CH}_3 \). Let us examine the carbon at position 2 (C-2):

• Group 1: \(-\text{H}\) (Hydrogen atom)

• Group 2: \(-\text{OH}\) (Hydroxyl group)

• Group 3: \(-\text{CH}_3\) (Methyl group)

• Group 4: \(-\text{CH}_2\text{CH}_3\) (Ethyl group)
All four groups attached to C-2 are distinctly different from one another. Therefore, C-2 is a chiral center. This makes the molecule non-superimposable on its mirror image, confirming that Butan-2-ol is chiral.

Step 3: Analysis of 1-Bromobutane and 2-Bromopropane.

• 1-Bromobutane (\(\text{CH}_2\text{Br}-\text{CH}_2-\text{CH}_2-\text{CH}_3\)): Every carbon atom in this chain is bonded to at least two hydrogen atoms. Since two bonds lead to identical atoms (H), none of these carbons are chiral.

• 2-Bromopropane (\(\text{CH}_3-\text{CHBr}-\text{CH}_3\)): The central carbon is bonded to one H, one Br, and two identical methyl groups. Just like Propan-2-ol, the presence of two identical groups makes it achiral.